1. Field of the Invention
The present invention is directed to a filter having a multi-cellular structure. In particular, the present invention is directed to a wall-flow filter for trapping diesel exhaust particulates and methods for manufacturing such filters.
2. Technical Background
Wall-flow filters are used in the purification of diesel exhaust. Typically, such diesel particulate filters are made of cordierite or silicon carbide and include a honeycomb body having thin interconnecting porous walls which are arranged and configured to form parallel cells longitudinally extending between the end faces of the structure. Alternating cells on one end face of the honeycomb are plugged with a ceramic filler material to form a “checkerboard” pattern. The pattern is reversed on the opposite side so that the ends of each cell are blocked at only one end of the structure. When diesel exhaust gas enters the filter through one end face (i.e., inlet end), it is forced to pass through the thin porous walls and exits through the opposite end face (i.e., outlet end) thereby filtering particulates carried in the exhaust gas.
Various filters having honeycomb structures are known in the art. U.S. Pat. Nos. 4,329,162; 4,415,344; 4,416,676; 4,417,908; and 4,420,316 discuss cordierite wall-flow diesel particulate filter designs. U.S. Pat. No. 5,914,187 discusses silicon carbide wall-flow diesel particulate filters. U.S. Pat. No. 6,696,132 and U.S. Pat. No. 6,843,822 disclose honeycomb structures for a filter in which the cells have non-equal cross sections, the larger cross section of the inlets allowing increased capacity for storing carbonaceous soot and ash particulates.
The application of such a filter in the exhaust gas stream causes a pressure drop in the gas that is filtered therethrough. Thus, presence of the filter results in a fuel penalty since additional work must be done by the engine to push the exhaust gas through the filter. Since it is desirable to minimize such parasitic losses, filter designs having low pressure drop are preferred. The pressure drop may be minimized by making the ceramic walls of the filter thin. A disadvantage with this approach is that, for a given soot loading level, a regeneration event will result in higher maximum temperatures for a filter with thin walls. This can result in damage to the filter, including catalyst deactivation, or cracking, or even melting of the filter.
In particular, as the exhaust passes through the filter, particulate matter (i.e., carbon soot and ashes) accumulates on the wall of the cells or in the pores of the wall and forms a soot layer. These particulates decrease the effective diameter of the cells and further contribute to the pressure drop across the filter, thereby increasing the back pressure that acts against the engine exhaust. The carbon soot can be burned off during a regeneration process in which the filter is heated sufficiently to initiate combustion of the carbon soot layer. Normally, during regeneration, the temperature in the filter rises from about 400-600° C. to a maximum of about 800-1000° C. Under certain circumstances, a so-called “uncontrolled regeneration” can occur when the onset of combustion coincides with, or is immediately followed by, high oxygen content and low flow rates in the exhaust gas. During an uncontrolled regeneration, the combustion of the soot may produce temperature spikes within the filter which can thermally shock and crack, or even melt, the filter.
Thus, the filters must be sufficiently durable to withstand such high temperatures experienced during regeneration. Correspondingly, whereas reduction in the wall thickness of the cells can reduce back pressure, this solution is not optimal in view of the fact that the durability is reduced as a result. Further, whereas increased wall thickness of the cells is desirable to increase the durability of the filter, such increased wall thickness also undesirably increases the backpressure.
In view of the above, there exists an unfulfilled need for a ceramic wall-flow filter that has thin filtration wall so as to minimize the pressure drop, but that has the durability to avoid potential damage during regeneration processes.